A thermosensitive recording process is a recording process wherein a thermosensitive recording head (hereinafter referred to simply as a head) is heated in accordance with input signals to cause a fusion contact between a color former and a color developer on an image receiving sheet (thermosensitive recording paper) in contact with the head, whereby color images can be obtained. The thermosensitive recording process has a recording speed corresponding to the quantity of information capable of being transmitted through a telephone circuit. This process is a primary color formation system which requires neither development nor fixing, and causes very little wear of the head. Because of these advantages, the process has been rapidly spreading to applications to information processing equipment such as printers, facsimile machines, etc.
With rapid development of various types of office devices and the variety of their uses, there is a demand for a thermosensitive recording sheet capable of meeting each particular requirement. For example, as a thermosensitive recording sheet capable of coping with the speed up of the recording device, a demand has arisen to develop a thermosensitive recording sheet capable of providing a clear image with high density even when using only a small amount of printing energy.
It has been recognized that not only thermosensitive recording layers but also supports must be examined to meet the above demand, and the use of synthetic resin films as the support in place of conventional natural paper has been increased.
For example, JP-A-2-70479 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") (U.S. Pat. No. 4,996,182) discloses a thermosensitive recording sheet wherein a biaxially stretched resin film layer having fine voids and a fine void content of 40 to 100 cc/100 g is used as a constituent element of the support for the thermosensitive recording layer, and a thermosensitive recording sheet wherein said biaxially stretched resin film layer is laminated with a film layer comprising the same material as that of the film or a different material from that of the film.
These thermosensitive recording sheets wherein the biaxially stretched film meeting the demand of only voids is used as a constituent element of the support can provide clear images with high density. However, since the surface strength thereof is low, there is a disadvantage that when the sheet is supercalendered to smooth the surface thereof after coating a thermosensitive layer, the coated thermosensitive layer is peeled off.
JP-A-59-148693, JP-A-61-279589, JP-A-62-282970, JP-A-63-99984 and JP-A-63-299976 disclose thermosensitive recording papers using a resin film containing an inorganic fine powder. These thermosensitive recording papers have good surface strength, but none of them can provide a clear image with high density.
Improvements in the high-speed printing of the thermosensitive recording devices have been made in a short time in recent years, and thermal dye transfer type image recording sheets capable of multiple transfer as described in JP-A-63-222891 have also needed to be able to make gradation recording of tone density even at a narrow pulse width.
A thermal dye transfer type image recording process is carried out with a transfer material (ink ribbon) comprising a support having thereon a coloring material layer containing a sublimable or vaporizable dye, which is heated to sublime or vaporize the dye contained in the coloring material layer, and the dye is deposited on an image receiving recording sheet, whereby a dye image can be formed.
As shown in FIG. 1, a transfer material 1 comprising a base 4 having thereon a coloring material layer 5 and an image receiving sheet 2 comprising a support 7 having thereon an image receiving layer 6 are put between a drum 8 and a heat surface 3, and the coloring material layer 5 is heated by means of a head surface capable of being controlled by electric signals, such as a thermal head. A dye contained in the coloring material layer 5 is sublimed or vaporized and deposited on the image receiving layer 6, whereby the thermal dye transfer type image recording can be effected.
The material of the image receiving layer 6 varies depending on the types of coloring materials to be deposited thereon. For example, when the coloring material is a hot-melt type, the support 7 itself may be used as the image receiving layer. When the coloring material is a sublimable disperse dye type, a high-molecular material coat layer such as a polyester coat layer can be used as the image receiving layer.
The support 7 of a conventional image receiving sheet 2 has an uneven thickness and an uneven surface, and hence the surface of the image receiving layer 6 itself has a roughness of 5 to 15 .mu.m and waviness of 10 to 20 .mu.m per mm. This roughness or waviness can be somewhat improved by supercalendering the surface of the image receiving layer 6. However, there is a limit to the degree of the improvement. For example, the surface of a conventional image receiving layer still has a roughness of at least 3 to 5 .mu.m or waviness of at least 10 .mu.m per mm. Accordingly, the coloring material (the hot-melt type as well as the sublimable dye) to be transferred from the coloring material layer 5 can not be correctly transferred according to image signals, and a disorder in image quality, such as unclearness of dots or failure in dots is caused. Further, intermediate tone suffers from roughness.
The supports used include paper, opaque synthetic paper comprising a stretched film of a propylene resin containing inorganic fine powder (as described in JP-B-46-40794 (the term "JP-B" as used herein means an "examined Japanese patent publication") and U.S. Pat. No. 4,318,950) and coated synthetic paper obtained by coating the surface of a transparent polyethylene terephthalate film or a transparent polyolefin film with an inorganic compound such as silica or calcium carbonate together with a binder to increase whiteness and dyeability.
However, when considering the condition (e.g., duplicability, pencil writeability, preservability) of the image receiving sheets after thermal dye transfer type image recording, synthetic paper obtained by stretching a polyolefin resin film containing inorganic fine powder to thereby form many microvoids therein and is preferred from the viewpoints of strength, dimensional stability and close contact with a printing head (see, JP-A-60 245593, JP-A-61-112693 and JP-A-63-193836).
In such synthetic paper obtained by stretching a polyolefin resin film, microvoids are formed in the interior of the film by stretching the film at a temperature of lower than the melting point of the polyolefin resin to impact opacity and soft feeling and to improve contact with a printing head, feedability and dischargeability.
However, improvements in high-speed printing of thermosensitive recording devices have been made in a short time in recent years, and thermal dye transfer type image recording sheets capable of multiple transfer as described in JP-A-63-222891 have also required that a gradation recording of tone density can be made even at a narrow pulse width.
Although the content of the inorganic fine powder can be reduced to increase the surface smoothness of synthetic paper because printing density is increased with an increase in smoothness, the volume of voids in the film is reduced by stretching. As a result, the cushioning effect of synthetic paper is reduced. Accordingly, the density of an image on the thermal dye transfer type image receiving sheet using this synthetic paper as the support is lowered as demonstrated in Comparative Example 1 of JP-A-63-222891.